EXPERT ANALYSIS FROM THE JOURNAL OF THORACIC AND CARDIOVASCULAR SURGERY
Advances in gene editing are pushing the possibility of raising pigs for organs that may be transplanted into humans with immunosuppression regimens comparable to those now used in human-to-human transplants, coauthors James Butler, MD, and A. Joseph Tector, MD, PhD, stated in an expert opinion in the February issue of the Journal of Thoracic and Cardiovascular Surgery ( 2017;153:488-92 ).
Developments in genome editing could bring new approaches to management of cardiopulmonary diseases, Dr. Butler and Dr. Tector noted. “Recently, cardiac-specific and lung-specific applications have been described, which will allow for the rapid creation of new models of heart and lung disease,” they said. Specifically, they noted gene targeting might eventually offer a way to treat challenging genetic problems “like the heterogeneous nature of nonsquamous cell lung cancer.”
Dr. Butler is with the department of surgery at Indiana University, Indianapolis, and Dr. Tector is with the department of surgery at the University of Alabama at Birmingham.
Driving this research are advances in a genome editing approach known as CRISPR, or clustered regularly interspaced short palindromic repeats, along with understanding of the CRISPR-associated Cas9 protein. “CRISPR gene-targeting is easier to use than previous tools because the Cas9 nuclease is not tethered to the gene targeting element; rather, it binds to a polymerase chain reaction primer-like single guide RNA,” Dr. Butler and Dr. Tector said. They called CRISPR technology “one of the greatest medical breakthroughs from prokaryotes since Alexander Fleming noticed a contaminant strain of Penicillium notatum clearing Staphylococcus cultures.”
CRISPR technology has been used in developing multiple gene knockout pigs and neutralizing three separate porcine genes that encode human xenoantigens in a single reaction, leading to efficient methods for creating pigs with multiple genetic modifications.
According to the website of the Broad Institute of MIT and Harvard, Cambridge, Mass., where researchers perfected the system to work in eukaryotes, CRISPR works by using short RNA sequences designed by researchers to guide the system to matching sequences of DNA. When the target DNA is found, Cas9 – one of the enzymes produced by the CRISPR system – binds to the DNA and cuts it, shutting the targeted gene off.
“By facilitating high-throughput model creation, CRISPR has elucidated which modifications are necessary and which are not; despite the ability to alter many loci concurrently, recent evidence has implicated three porcine genes that are responsible for the majority of human-antiporcine humoral immunity,” Dr. Butler and Dr. Tector wrote.
Those genes are the Gal[alpha]1-3 epitope (Gal-alpha), CMAH and B4GaINT2 genes. “Each of these three genes is expressed in pigs but has been evolutionarily silenced in humans,” the coauthors added.
When those three genes are silenced in pigs, “measures of human-antiporcine humoral immunity are reduced to levels comparable with allograft transplantation standards,” they said ( Xenotransplantation. 2015 Jan-Feb;22:20-31). “This raises the possibility that porcine-to-human xenotransplantation may be clinically viable with the addition of conventional immunosuppressive regimens.”
While CRISPR genome editing has yet to reach its full potential, researchers and clinicians should pay attention, according to Dr. Butler and Dr. Tector.
More recent modifications of CRISPR technology have shown promise in not just knocking out or turning off specific genes, but rather guiding directed replacement of genes with researcher-designed substitutes. This can enable permanent transformation of functional genes with altered behavior, according to the Broad Institute website.
Dr. Tector disclosed he has received funding from United Therapeutics and founded Xenobridge with patents for xenotransplantation. Dr. Butler has no relevant financial relationships to disclose.